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Abstract

In this paper, we use the transfer matrix method to calculate the optical absorptance of vertically-aligned silicon nanowire (SiNW) arrays. For fixed filling ratio, significant optical absorption enhancement occurs when the lattice constant is increased from 100nm to 600nm. The enhancement arises from an increase in field concentration within the nanowire as well as excitation of guided resonance modes. We quantify the absorption enhancement in terms of ultimate efficiency. Results show that an optimized SiNW array with lattice constant of 600nm and wire diameter of 540nm has a 72.4% higher ultimate efficiency than a Si thin film of equal thickness. The enhancement effect can be maintained over a large range of incidence angles.

Optical properties of the SiNW array with varying lattice constant. In the left column, (a), (b), and (c) are the reflectance, transmittance, and absorptance, respectively, in the SiNW array with lattice constant 100nm and 500nm. The optical properties of a silicon thin film are plotted for comparison. In the right column, (d), (e), and (f) are the reflectance, transmittance, and absorptance, respectively, in the SiNW array with lattice constants 100nm, 150nm, and 200nm. The optical properties of the Si thin film are also plotted.

(a) Characteristic line shape of a guided resonance at 1.197eV (b) Top view of electric field energy density distribution on the end surface of the SiNW array at the resonance shown in (a). (c) Side view of electric field energy density distribution inside the nanowires at the same resonance.